A modern trend in such systemns is towards the use of electronics, and especially to the use of time division multiplex (TDM) and pulse code modulation (PCM). One such system has been described in British Patent Specification No. 2067050B (Western Electric), and the system described therein uses a number of switching networks each of which serves a number of subscribers' lines connected to a common central switching unit.
An object of the present invention is to extend the usefulness of such systems.